Separation method and separation apparatus of isotopes from gaseous substances

ABSTRACT

A isotopic gas of high purity is obtained at low cost, wherein for example, in a case of separating  13 CH 4  from methane gas and obtaining  13 CH 4  of high purity, the enrichment of  13 CH 4 , using the difference in adsorption of  12 CH 4  and  13 CH 4  onto an adsorbing material, is performed up to a certain concentration, and thereafter,  13 CH 4  is enriched by a separation method by distillation that uses the difference in boiling point.

BACKGROUND OF THE INVENTION

This invention concerns a method and a device for separating an isotopicgas, and particularly concerns an art that can be effectively applied toa method and a device for separating an isotopic gas efficiently at lowenergy consumption (or low electric power consumption).

As is well known, materials that exist in nature contain isotopes atcertain proportions. For example, a carbon atom with a mass number of 13(¹³C) exists as an isotope of a carbon atom with a mass number of 12(¹²C), and for example, with methane gas that is collected as naturalgas, methane gas of a mass number of 17 (¹³CH₄) exists at a proportionof 1.1 volume % in addition to methane gas of a mass number of 16(¹²CH₄). Various industrial fields of application exist for isotopes,and for example in the medical field, carbon with a mass number of 13(¹³C) is used. Efficient medical examination methods, etc., are forexample made possible by the use of ¹³C.

Since isotopes hardly differ from each other in chemical properties,generally, it is necessary to pay a large cost for building andoperating an apparatus for separating isotopes of different mass numbersfrom nature. For carbon isotopes, an art of separating the isotopic gas(¹³CH₄ or ¹³CO) contained in methane gas or carbon monoxide gas bydistillation (low-temperature fine distillation) is known.

With a method of separating isotopes by distillation, the isotopes areseparated by making use of the difference in the boiling points of theisotopes. With a distillation separation method, a device called adistillation column is used. A distillation column has a structurewherein the upper part is cooled and the lower part is heated. Forexample in the case of methane, when methane gas is introduced into thedistillation column and the temperature distribution inside thedistillation column is adjusted finely, a low-boiling-point component(¹²CH₄) collects at the upper part of the distillation column since itdoes not liquefy readily, and a high-boiling-point component (¹³CH₄)collects at the lower part of the distillation column since it liquefiesreadily. Methane gas is thus separated into ¹³CH₄ and ¹²CH₄.

However, with the above-described distillation method, there areproblems due to the boiling points of the treated gases being extremelylow in temperature.

As is clear from the principles, with a distillation separation method,the treatment temperature should be controlled in the vicinity of theboiling points of the treated gases. Generally, a substance that is in agaseous state at room temperature and atmospheric pressure conditions ofapproximately 1 atm and 300K has an extremely low boiling point, and,for example, the boiling point of methane gas is approximately 111K. Avast amount of cooling energy is required to control a distillationcolumn at such a cryogenic temperature. A large amount of cooling energyis consumed especially at an initial stage of distillation at which theabundance ratio of an isotope is small with respect to another isotopicgas since a large amount of gas must be controlled at a cryogenictemperature.

Also with a distillation separation method, since the isotopic gas thatis to be enriched is low in recovery, a large amount of gas in the stateprior to treatment (mixed gas in the sate prior to enrichment) must beintroduced into the distillation column. For example, in a case where¹³CH₄ and ¹²CH₄ are to be separated by a distillation separation methodusing a distillation column with practical height, if a mixed gas of¹³CH₄ and ¹²CH₄ that contains 1.1 volume % of ¹³CH₄ is introduced intothe distillation column, approximately 1 volume % of ¹³CH₄ will still becontained in the ¹²CH₄ that is discharged from upper part of thedistillation column. The low recovery of the isotopic gas that is to beenriched is inevitable with the practical range of distillation columnheight or number of distillation columns.

Also in relation to the above-described low recovery of the isotopic gasthat is to be enriched, the unavoidable increase in the proportion thatequipment for the low enrichment stage occupies among the entire systemfor distillation separation should be noted. This means that in a caseof separating an isotopic gas that is contained at a minute proportionby distillation separation, a high proportion of the energy necessaryfor cooling and heating is consumed at the low enrichment stage.

A distillation separation method also has the problem that a longstartup time is required for attaining a concentration distributionnecessary for steady-state operation from the start of supply of treatedgas and energy into the distillation column (time in year units may berequired depending on the scale of the plant). This is also a factorthat increases operation costs.

As an art besides methods using distillation, there is the art proposedin Japanese unexamined Patent Publication No. Hei 10-128071. With thisart, zeolite, having a pore diameter close to the molecular diameter ofthe isotopic gas, is used and the differences in adsorption onto zeoliteof the isotopic gases that differ in mass number are used to separatethe isotopic gases. Also, Japanese unexamined Patent Publication No.2001-219035 discloses an art of using a zeolite-based adsorbing materialto separate ¹²CO and ¹³CO. This art uses the zeolite-based adsorbingmaterial property of selectively adsorbing ¹³CO more readily in order toseparate ¹²CO and ¹³CO.

With the above-described separation of isotopic gases using adsorption,a vast amount of cooling energy is not required as in the case ofdistillation separation. However, with this method of separatingisotopic gases using adsorption, the enrichment efficiency at the finalstage of enrichment is not necessarily good. This becomes a problem in acase where an isotopic gas of high purity is to be obtained.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an art for separatingisotopic gases that does not require a vast amount of input energy andenables the shortening of the startup period. Another object of thisinvention is to provide an art for separating an isotopic gas thatexists in minute amounts and enriching the isotopic gas to high purityefficiently and at low cost.

First, the terms used in this Description shall be described. “Isotopicgas of low mass number” refers to an isotopic gas having an atom ofsmaller mass number as its component. “Isotopic gas of high mass number”refers to an isotopic gas having an atom of higher mass number as itscomponent. For example in the case of methane gas, ¹²CH₄ is the isotopicgas of low mass number and ¹³CH₄ is the isotopic gas of high massnumber.

“Molecular gas” refers to a gas, such as methane, with which thecomponents are molecules. “Atomic gas” refers to a gas, such as argon,with which the components are atoms. “Mixed gas” refers to a gas to betreated that contains a plurality of types of isotopic gases. A mixedgas may contain other impurities. Examples of mixed gases that can beused include methane gas, which is separated from natural gas andcontains ¹²CH₄ and ¹³CH₄ at a volume ratio of 0.99:0.01, and carbonmonoxide gas, containing ¹²CO and ¹³CO at a volume ratio of 0.99:0.01.

“First gas” refers to the isotopic gas of low mass number and theisotopic gas of the first gas that is to be separated is the isotopicgas of high mass number. Though generally the term, “isotope,” is usedin a manner such that atoms or molecules consisting of the same elementsthat differ in mass numbers are mutually called isotopes, with the term,“isotope,” in the description of this invention, the gas of high massnumber that is to be separated is referred to as the “isotopic gas.”Though the abovementioned first gas and the isotopic gas in theabovementioned expression are isotopes of each other and, broadlyspeaking, it is thus possible to refer to the first gas as an isotopicgas as well, with the present Description, the isotopic gas of low massnumber is referred to by the term, “first gas,” and the isotopic gas ofhigh mass number is referred to by the term, “isotopic gas.”

For separation and enrichment of an isotopic gas, this invention makesuse of a method using adsorption in a low enrichment stage andthereafter makes use of a method that uses distillation to enrich theisotopic gas further. According to findings obtained by the presentinventors, although a method using adsorption is comparatively low inconsumption energy, it is unsuited for enrichment of an isotopic gas ata stage at which the concentration has become high. On the other hand,with a method using distillation, the scale will not become very largeand the consumption energy will not be much of a problem ifconcentration has progressed to some degree. Moreover, in comparison toa method using adsorption, a method using distillation is more suited tothe enrichment of an isotopic gas at a stage at which the concentrationhas become high. The two methods are thus combined to perform separationand enrichment of an isotopic gas using adsorption at a low enrichmentstage and then switching to separation and enrichment of the isotopicgas by distillation at a stage at which enrichment has progressed tosome degree. The overall consumption energy can thus be reduced incomparison to the prior arts and yet an isotopic gas of highconcentration (isotopic gas of high purity) can be obtained readily.

This invention makes use of either of two phenomena for separating anisotopic gas by adsorption. One is the phenomenon that when a mixed gascontaining two or more types of isotopic gases is made to contact anadsorbing material that meets specific conditions, it is more difficultfor the isotopic gas to become adsorbed and become desorbed incomparison to the first gas.

By making use of this phenomenon and making a mixed gas containing twoor more types of isotopic gases contact an adsorbing material that meetsspecific conditions, the concentration of the isotopic gas can be madehigh at the beginning of recovery of the contacted gas. For example,when the abovementioned mixed gas is made to flow when the adsorbingmaterial that meets specific conditions is in a state in which theisotopic gas that is to be separated is not adsorbed, the molecules ofthe first gas become captured first and then the molecules of theisotopic gas become captured at a delayed timing. Thus with the mixedgas at the initial stage at which its flow is started, the proportion ofthe isotopic gas will be relatively greater than the proportion of thefirst gas. A mixed gas, with which the isotopic gas is separated andenriched, can thus be obtained.

Also by making use of the above-described phenomenon and performingdesorption with the adsorbing material that meets the specificconditions being in a state where two or more types of isotopic gasesare adsorbed and recovering the desorbed gas after the elapse of apredetermined time from the start of desorption, a gas, with which theproportion of the isotopic gas, which is delayed in desorption, has beenmade high, can be obtained. With the present Description, “desorption”refers to the separation of an adsorbed substance from a surface towhich the substance is adsorbed.

The other phenomenon that this invention makes use of is the phenomenonthat when a mixed gas containing two or more types of isotopic gases ismade to contact an adsorbing material that meets specific conditions,the isotopic gas becomes adsorbed more readily than the first gas.

By making use of this phenomenon and contacting a mixed gas with aspecific adsorbing material and causing the adsorbed gas components todesorb thereafter, the proportion of the isotopic gas among the desorbedcomponents becomes higher than that prior to adsorption. Separation andenrichment of the isotopic gas can thus be performed using thisphenomenon.

In outline, this invention provides the following. A first mode of thisinvention provides in an isotopic gas separation method for separating,from a mixed gas containing a molecular or atomic first gas, an isotopicgas of the abovementioned first gas, an isotopic gas separation methodcomprising: one of either a first treatment procedure, in turncomprising the steps of: supplying the abovementioned mixed gas to a gasinlet of an adsorption chamber; and taking out the isotopic gas of theabovementioned first gas that flows out from a gas outlet of theabovementioned adsorption chamber from the start of supplying of theabovementioned mixed gas to the point of elapse of a predetermined time;or a second treatment procedure, in turn comprising: a first step ofsealing the abovementioned mixed gas in an adsorption chamber; a secondstep of making the abovementioned mixed gas flow out from theabovementioned adsorption chamber after the abovementioned first step;and a third step of taking out the isotopic gas of the abovementionedfirst gas after the elapse of a predetermined time from the start of theabovementioned second step; and further comprising: a treatmentprocedure of enriching, by distillation, the abovementioned isotopic gasof the abovementioned first gas that has been taken out.

With the above-described first mode of this invention, the gas thatflows out from the interior of the adsorption chamber takes out, at astage at which the adsorption and desorption onto the adsorbing materialof the first gas is closer to the equilibrium state, but the adsorptionand desorption of the isotopic gas does not become the equilibrium stateyet. Therefore, a mixed gas with which the concentration of the isotopicgas has been increased is obtained. Then at a stage at which theconcentration of the isotopic gas has become high to some degree,further separation of the isotopic gas and the first gas is performed bydistillation. In a region in which the concentration of the isotopic gasis low, the separation of isotopic gas by use of adsorption iscomparatively high in efficiency and is energy-saving in comparison todistillation. The problem of large consumption energy at the lowenrichment stage in the prior-art method by distillation is thusresolved. Also, since in a region in which the concentratedconcentration has become high, distillation, which is better inseparation efficiency (enrichment efficiency) in comparison to a methodusing adsorption, is used, an advantage is provided in terms ofobtaining isotopic gas of high purity.

Also with the above-described first mode of this invention, byrecovering gas, which flows out of the adsorption chamber from a stateof being sealed inside the adsorption chamber, after the elapse of apredetermined time from the start of outflow, gas, which has been madehigh in the concentration of the isotopic gas that is delayed indesorption, can be recovered. The same effects as those described abovecan also be obtained in this case by switching to distillation at astage at which the concentration of the isotopic gas has become high tosome degree.

The switching from separation using adsorption to separation bydistillation is performed at a stage at which the concentration, in therecovered gas, of the isotopic gas that is to be separated exceeds thenatural abundance ratio and preferably at a stage at which theconcentration of the isotopic gas has become 10 to 80 volume % and morepreferably at a stage at which the concentration of the isotopic gas hasbecome 10 to 50 volume %. When the concentration of the isotopic gas tobe separated is less than 10 volume %, separation using distillationwill be relatively high in cost. However, in a case where an adsorptionseparation equipment is to be added to an existing distillationequipment in order to answer demands for increased production, etc., thecost can be made lower than in a case where a distillation equipment isto be installed additionally even if the concentration at which theabovementioned switch is made is less than 10 volume %. When theconcentration of the isotopic gas that is to be separated exceeds 50volume %, the method of separation using adsorption becomes low inseparation efficiency and the merit thereof falls.

As the material for adsorbing the isotopic gas in the above-describedfirst mode of this invention, activated carbon, A-type zeolite, or acomplex may be used. Examples of a complex that can be used include athree-dimensional metal complex of dicarboxylic acid, etc. As the mixedgas, methane gas or ammonia gas may be used.

With the above-described first mode of this invention, after performingseparation of the isotopic gas, it is preferable to perform aregeneration process of heating the adsorbing material under areduced-pressure atmosphere and removing the adsorbed substances andthen perform the isotopic gas separation process again in a repeatingmanner.

As the adsorption of the isotopic gas progresses, saturation occurs anda state of equilibrium is reached as adsorption and desorption becomenearly equivalent. When this occurs, the proportion of the isotopic gasin the mixed gas will be the same before and after contact with theadsorbing material and the separation efficiency of the isotopic gasthat is to be separated drops. Thus after performing adsorption of theisotopic gas to be separated, by removing the substances that had becomeadsorbed onto the adsorbing material and recovering the adsorbingability of the adsorbing material, the separation of the isotopic gascan be performed again. By thus performing the isotopic gas separationprocess and the adsorbing material regeneration process repeatedly,efficient separation of the isotopic gas can be performed.

A second mode of this invention provides in an isotopic gas separationmethod for separating, from a mixed gas containing a molecular or atomicfirst gas, an isotopic gas of the abovementioned first gas, an isotopicgas separation method comprising the steps of supplying theabovementioned mixed gas to a gas inlet of an adsorption chamber;stopping the abovementioned supply after the elapse of a predeterminedtime from the start of supply of the abovementioned mixed gas;depressurizing the interior of the abovementioned adsorption chamber ina relative manner and taking out the abovementioned mixed gas that hadbecome adsorbed inside the abovementioned adsorption chamber; andenriching, by distillation, the isotopic gas of the abovementioned firstgas contained in the abovementioned mixed gas that has been taken out.

A third mode of this invention provides in an isotopic gas separationmethod for separating, from a mixed gas containing a molecular or atomicfirst gas, an isotopic gas of the abovementioned first gas, an isotopicgas separation method comprising the steps of supplying theabovementioned mixed gas to a gas inlet of an adsorption chamber;stopping the abovementioned supply after the elapse of a predeterminedtime from the start of supply of the abovementioned mixed gas; makingcarrier gas flow inside the abovementioned adsorption chamber and takingout, along with the abovementioned carrier gas, the abovementioned mixedgas that had become adsorbed inside the abovementioned adsorptionchamber; and enriching, by distillation, the isotopic gas of theabovementioned first gas contained in the abovementioned mixed gas thathas been taken out.

With the above-described second and third modes of this invention, moreof the isotopic gas, which becomes adsorbed readily onto the adsorbingmaterial in the adsorption chamber, becomes adsorbed in the adsorptionchamber, and by recovering the adsorbed component, the isotopic gas thathas become relatively higher in concentration can be obtained.Separation and enrichment of the isotopic gas can thereby be performed.Furthermore, by performing the abovementioned enrichment usingadsorption at an early stage of enrichment at which the concentration ofthe isotopic gas is low and switching to enrichment using distillationfrom a stage at which the enrichment has progressed, low energyconsumption (for example, low power consumption) and high purity at highefficiency can be realized.

With the above-described second and third modes of this invention, aporous material may be used as the material for adsorbing the isotopicgas. In particular, zeolite, activated carbon, silica gel, or aluminamay be used as the material for adsorbing the isotopic gas. Also, carbonmonoxide gas may be selected as the mixed gas. In a case wherecopper-ion-exchanged zeolite is used as the adsorbing material, ammoniagas may be selected as the mixed gas. In this case, ¹⁴NH₃ and ¹⁵NH₃ areseparated and enrichment of ¹⁵NH₃ can be performed.

With the second and third modes of this invention, the switching fromseparation using adsorption to separation using distillation is alsoperformed at a stage at which the concentration, in the recovered gas,of the isotopic gas that is to be separated exceeds the naturalabundance ratio and preferably at a stage at which the concentrationbecomes 10 to 80 volume % and more preferably at a stage at which theconcentration becomes 10 to 50 volume %.

Also with the above-described second and third modes of this invention,faujasite, pentasil zeolite, mordenite, or A-type zeolite is preferablyused as zeolite.

This invention can also be put to practice in the form of an isotopicgas separation device. In this case, the device has an arrangement ormeans for executing this invention's isotopic gas separation methoddescribed above.

As has been described above, with the present invention, sinceenrichment at a low enrichment stage is performed by an adsorptionmethod and enrichment using distillation is performed at a stage oftransition into a stage of relatively high enrichment, the load placedon distillation equipment at the low enrichment stage can be lightened.The number of distillation columns of a plant as a whole can thus bereduced in comparison to a case where only distillation processes arecarried out. Or, the effect of reducing the number of enrichment stagesor lowering the height of a distillation column can be provided. In thiscase, since the absolute amount of holdup liquid (a flow with which thefirst gas in the liquefied condition is the main component) that isrefluxed within the distillation column can be reduced, the startupperiod until the attainment of the concentration distribution forsteady-state operation can be reduced. The operation cost can thus bereduced in comparison to the case where only a distillation process isused.

The above-described invention may be used to achieve low cost bycombining an adsorption process with a part of a distillation processthat is already in operation. Examples of modes of practice includeproviding, in a plant for isotope separation by distillation with whichincreased production of isotopic gas is demanded, an additionaladsorption separation equipment at a part at which an enrichment processof a low enrichment stage is performed and making a part of the isotopicgas enrichment process be shouldered by an adsorption process or makinga part of the treatment be shouldered by the adsorption process in aparallel manner to increase the productivity of the low enrichment stageprocess. The advantage of alleviating the load placed on a distillationprocess of a low enrichment stage can be obtained with such a method aswell.

As an isotope separation process (isotope enrichment process) usingadsorption, any of the above-described arts of isotope separation byadsorption may be selected as suited according to the gas or adsorbingmaterial.

Though the effects of this invention can be provided most highly byputting the isotope separation process (isotope enrichment process)using adsorption to use at an early stage of an isotope separationprocess, the use is not limited necessarily to an early stage of anisotope separation process as long as the economy of a distillationprocess in a stage of relatively low enrichment can be improved.

By above mentioned invention, an isotopic gas separation art that doesnot require a vast amount of input energy and enables shortening of thestartup period is provided. This invention also provides an art forperforming efficient and low-cost separation of an isotopic gas, whichexists in minute amounts, and enrichment of the isotopic gas to highpurity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram, showing an example of a system for carrying outthis invention's isotopic gas separation method.

FIG. 2 is a flowchart, showing an example of a treatment procedure of anembodiment to which this invention's isotopic gas separation method isapplied.

FIG. 3 is a flowchart, showing an example of a treatment procedure of anembodiment to which this invention's isotopic gas separation method isapplied.

FIG. 4 is a diagram, showing an example of a system for carrying outthis invention's isotopic gas separation method.

FIG. 5 is a flowchart, showing an example of a treatment procedure of anembodiment to which this invention's isotopic gas separation method isapplied.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of this invention shall now be described in detail based ondrawings. However, this invention may be practiced in various differentmodes and should not be interpreted as being limited to thesedescriptions of the embodiments. Throughout all of the embodiments, thesame elements shall be provided with the same numbers.

First Embodiment

An example of an isotopic gas separation method arranged on the basis ofthe findings given above shall now be described. The isotopic gasseparation method using adsorption in this embodiment makes use of thephenomenon in which a specific isotopic gas is less readily adsorbedonto and desorbed from a specific adsorbing material in comparison to afirst gas. The separation of isotopic gas using adsorption in thisembodiment makes use of the phenomenon that in the process of passing amixed gas through an adsorption chamber in which is installed theadsorbing material, the gas that is passed through in an early stagecontains the isotopic gas at high concentration since the isotopic gasis less readily adsorbed in comparison to the first gas.

With the present embodiment, ¹²CH₄ is used as the first gas, ¹³CH₄ isused as the isotopic gas to be separated, and activated carbon is usedas the adsorbing material.

FIG. 1 is a diagram, showing an example of a system for carrying outthis invention's isotopic gas separation method. The system shown inFIG. 1 comprises a flow regulating device 400, flow regulating device401, piping 101, valve 102, piping 103, valve 104, activated carbon 105,adsorption chamber 107, temperature regulating device 108, piping 109,recovery pump 110, valve 111, valve 112, exhaust pump 113, recovery tank114, valve 115, exhaust pump 116, valve 117, flow regulating device 118,piping 120, distillation column 131, piping 132, piping 133,distillation column 141, piping 142, and piping 143.

Helium (He) gas, to be used as carrier gas, is introduced from piping101. High-purity methane (CH₄) gas is introduced from piping 103.Needless to say, high-purity methane contains both ¹²CH₄ and ¹³CH₄.Adsorption chamber 107 has a structure enabling the interior to bemaintained at a reduced pressure state. Adsorption chamber 107 can beheated or cooled to a predetermined temperature by means of temperatureregulating device 108 and the internal temperature can be adjustedarbitrarily. The interior of adsorption chamber 107 can be put in areduced pressure state by means of recovery pump 110 and exhaust pump113.

Activated carbon 105 functions as an adsorbing material. As activatedcarbon 105, activated carbon having an average pore diameter of 2 timesand preferably as close to 1 time the molecular diameter of methane isused. This is because it has been confirmed experimentally that when theaverage pore diameter of activated carbon is 2 times and preferably asclose to 1 time the molecular diameter of methane, ¹²CH₄ and ¹³CH₄ canbe separated efficiently.

An example of a method of producing activated carbon shall now bedescribed. As a raw material for activated carbon, a material selectedfrom among cellulose, cellulose compounds, polyimide, polyimidecompounds, and natural substances and artificial substances havingcellulose as the main component or a mixture of a plurality of suchmaterials may be used. For production, first the raw material is madeinto a powder and placed in a mold upon addition of a binder asnecessary. This is then pressurized to obtain material of apredetermined shape. Thereafter, the molded material is subject to heattreatment. The heat treatment is performed in two steps. First, heattreatment for carbonization is performed. This heat treatment isperformed for example under a nitrogen atmosphere and under thecondition of 1073K for 6 hours. The material is carbonized by this heattreatment. A second heat treatment is then performed. This heattreatment is performed for example under a carbon dioxide atmosphere andunder the condition of 1173K for 6 hours. Activation occurs and a changeto a porous state progresses in this second heat treatment. Though thechange to the porous state also occurs in the first heat treatment, thechange to the porous state proceeds further in the second heattreatment. By controlling the conditions of the second heat treatment,the density of pores and the pore diameter can be controlled. Since thecontrol conditions for the pore diameter and density of pores depend onthe raw material and the atmosphere, these must determined byexperiment.

Recovery tank 114 is a tank for recovering gas exhausted from adsorptionchamber 107. Distillation column 131 is a distillation column forseparating methane from helium, which is the carrier gas, bydistillation. Distillation column 131 may be a gas separation device,such as a PSA. The separation of ¹²CH₄ and ¹³CH₄ is carried out atdistillation column 141. Distillation column 141 is equipped withunillustrated temperature regulating devices at its upper part and lowerpart and has a function of collecting a high-boiling-point component atthe lower part of the distillation column, collecting alow-boiling-point component at the upper part of the distillationcolumn, and thereby separating the high-boiling-point component from thelow-boiling-point component. PSA is the abbreviation for Pressure SwingAdsorption.

In the following, an example of a process for separating and extracting¹³CH₄ from high-purity methane gas shall be described. In the following¹²CH₄ corresponds to being the first gas of this invention and ¹³CH₄corresponds to being the isotopic gas of the first gas of thisinvention.

The high-purity methane gas corresponds to being the mixed gascontaining the first gas and the isotopic gas of this invention.

FIG. 2 is a flowchart, showing an example of a treatment procedure of anembodiment to which this invention's isotopic gas separation method isapplied.

First, in a state where all valves are closed, exhaust pump 113 is madeto operate, valve 112 is opened, and adsorption chamber 107 is put in astate of reduced pressure. Valve 112 is then closed and valve 102 isthen opened to fill the interior of adsorption chamber 107 with heliumgas. Valve 102 is then closed and valve 112 is opened with exhaust pump113 being in operation to exhaust the helium gas inside adsorptionchamber 107. This series of operations is repeated a plurality of timesto remove impurities that exist in adsorption chamber 107 as much aspossible and regenerate activated carbon 105 at the same time.Adsorption chamber 107 is then put in a high vacuum state of 13 Pa orless. Also, valves 111 and 117 are closed and then valve 115 is openedwith exhaust pump 116 being in operation to bring the interior ofrecovery tank 114 to a high vacuum state. When the interior of recoverytank 114 has been brought to a high vacuum state, valve 115 is closed.

Separation of the isotopic gas is carried out from this state. Here, theseparation of ¹³CH₄, which is the isotopic gas of ¹²CH₄, is started(step 501). First, with valves 111 and 112 being closed, valve 102 andvalve 104 are opened and then valve 111 is opened to introducehigh-purity methane gas and helium gas into adsorption chamber 107 (step502). In this process, flow regulating devices 400 and 401 are adjustedto realize flow at a predetermined flow rate, and the opening of valve111 is adjusted to maintain the pressure inside adsorption chamber 107at a predetermined pressure. Also, temperature regulating device 108 ismade to operate to maintain the temperature inside adsorption chamber107 at a fixed value (for example, 278K). Flow regulating devices 400and 401 are adjusted so that the ratio of the flow rates of methane gasand helium gas will for example be 1:9.

The methane gas and helium gas that flow into adsorption chamber 107flow through the interior of adsorption chamber 107 and is recovered inrecovery tank 114 by means of recovery pump 110 (step 503). With theflow of methane gas that flows into adsorption chamber 107, since ¹²CH₄is more readily adsorbed by activated carbon 105 in comparison to theisotopic gas, ¹³CH₄, ¹²CH₄ begins to be adsorbed by the activated carbonfirst and ¹³CH₄ begins to be adsorbed by activated carbon 105 at adelayed timing. As a result, with the gas that is discharged fromadsorption chamber 107 to recovery tank 114, the concentration of ¹³CH₄is higher in comparison to that of ¹²CH₄ at the initial stage. As themethane gas is made to flow for some period of time, the adsorptionamount and desorption amount of ¹³CH₄ reach an equilibrium and the ratioof ¹²CH₄ to ¹³CH₄ in the methane gas that is discharged from adsorptionchamber 107 becomes substantially equal to the ratio of ¹²CH₄ to ¹³CH₄in the methane gas that flows into adsorption chamber 107.

Thus at a stage at which a predetermined time has elapsed, valve 111 isclosed and the take-out of flowing gas from adsorption chamber 107 isstopped (step 504). This time from the start of inflow to the stoppageof inflow of the methane gas into adsorption chamber 107 is set, forexample, to 200 seconds. The methane gas that is discharged fromadsorption chamber 107 during this period is high in the concentrationof ¹³CH₄. Exhaust gas (methane gas and helium gas), with which the ¹³CH₄concentration has been increased, is thus collected in recovery tank114. The exhaust gas that has been collected in recovery tank 114 issent from recovery tank 114 to distillation column 131 as suited by thefunction of flow regulating device 118. The separation of methane gasand helium gas is performed at distillation column 131. The methane gasis then sent to distillation column 141 via piping 133. The helium gasis recovered from piping 132.

After stoppage of inflow of the methane gas into adsorption chamber 107,valve 112 is opened with exhaust pump 113 being in operation to bringthe interior of adsorption chamber 107 to a high vacuum state. In thisprocess, temperature regulating device 108 may be controlled to heat theinterior of adsorption chamber 107 to enhance the desorption efficiency.The heating temperature is set, for example, to 373K. Valve 102 is thenopened to make helium gas flow into adsorption chamber 107. Theregeneration process of desorbing the ¹²CH₄ molecules and ¹³CH₄molecules that had become adsorbed onto activated carbon 105 is thusexecuted (step 505).

After the regeneration process, the judgment of repeating the process ofseparating ¹³CH₄ again is made (step 506), and the methane gas isintroduced inside adsorption chamber 107 again and the ¹³CH₄ separationprocess of the next cycle is performed. The ¹³CH₄ separation processusing activated carbon 105 and the regeneration process of activatedcarbon 105 are thus performed repeatedly. In conjunction with theseprocesses, methane gas, which has been made high in ¹³CH₄ concentration,and helium gas, which is the carrier gas, are collected in recovery tank114, and the collected gas is sent to distillation column 131 fromrecovery tank 114. The separation of helium gas and methane gas is thenperformed at distillation column 131. The separated methane gas is thensent to distillation column 141 from piping 133. If the ¹³CH₄ separationprocess is to be ended, a “no” judgment is made at step 506 and theseparation of the isotopic gas is ended (step 507). The respective stepsdescribed above may be executed automatically in accordance with apriorly prepared program using an unillustrated computer control device,etc.

Normally, steps 502 to 506 of FIG. 2 are repeated. Helium gas andmethane gas, which has been made high in ¹³CH₄ concentration, are thensent continuously via piping 120 to distillation column 131, at whichthe helium gas is separated. In the adsorption process, the methane gasthat has been made high in ¹³CH₄ concentration is then sent fromdistillation column 131 to distillation column 141.

In a stage prior to the distillation process, the concentration of ¹³CH₄in the methane gas is preferably 10 volume % or more. This is foravoiding the consumption of vast amounts of energy and the making of theequipment large in scale for the initial stage at the start ofseparation (start of enrichment) by the method of isotope separation bydistillation. Based on the findings of the present inventors, asignificant reduction in cost in comparison to the prior-art process ofseparation and enrichment by distillation alone can be achieved if theconcentration of ¹³CH₄ existing in the methane gas is 10 volume % ormore. The methane gas that has been sent to distillation column 141 issubject further to the separation of ¹²CH₄ and ¹³CH₄ there. Atdistillation column 141, the temperature of the interior is adjusted toa value near the boiling point of methane to set up a state under whichboth ¹²CH₄ and ¹³CH₄ will liquefy readily. When under this state, thelower part of distillation column 141 is heated slightly and the upperpart is cooled slightly, a state, in which ¹²CH₄, which is alow-boiling-point component in comparison to ¹³CH₄, gasifies morereadily due to the boiling point difference, is obtained under delicateconditions. That is, by fine adjustment of an unillustrated temperatureregulating device equipped in distillation column 141, a 0.03Kdifference in boiling point is used to set up a state in which thelow-boiling-point component gasifies readily and the high-boiling-pointcomponent does not gasify readily but liquefies readily. As a result,the low-boiling-point component (¹²CH₄) collects at the upper part ofdistillation column 141 and is discharged to the exterior from piping143. Meanwhile, the high-boiling-point component (¹³CH₄) collects at thelower part of distillation column 141 and is taken out via piping 142.With case of using the distillation column of practical height, it isimpossible to separate ¹²CH₄ and ¹³CH₄ completely and a considerableamount of ¹³CH₄ will be contained in the methane gas that is dischargedfrom piping 143.

The methane gas that is discharged from piping 143 may then be guided topiping 144 and mixed with the raw material methane gas and therebysubject to recycled use to improve the efficiency further. Also, thoughthe exhaust gas from adsorption chamber 107 was collected in recoverytank 114 once in the above description, the exhaust gas may be guidedintermittently to distillation column 131 directly without the use ofrecovery tank 114.

Though distillation column 141 is indicated as a distillation column forperforming the separation of ¹²CH₄ and ¹³CH₄ in FIG. 1, in practice,distillation columns may be disposed in more stages to performdistillation through a greater number of stages in accordance with thetargeted purity of ¹³CH₄. Also, the separation (enrichment) of ¹³CH₄ bydistillation is high in controllability. ¹³CH₄ of the desired purity canthus be obtained readily.

With the above-described ¹³CH₄ separation method (enrichment method),since the consumption of energy (for example, electric power) at the lowconcentration stage can be restrained, the entire system can be made lowin consumption energy (for example, power-saving) and can thus be madehigh in economy. The treatment speed is also high.

Especially with the method of the present embodiment, the enrichment of¹³CH₄, which theoretically uses only the distillation column whichrequires several thousand stages, can be simplified. In particular,since a distillation process, which consumes a large amount of energy(for example, electric power), is not used at a low concentration stageof the separation process in which a large amount of methane gas must beprocessed, the running cost for obtaining ¹³CH₄ of high purity can bereduced greatly in comparison to the prior art of using onlydistillation. The equipment cost can also be reduced since the equipmentcan be simplified.

The enrichment of isotopic gas by adsorption may also be carried out ina plurality of stages in order to obtain the necessary concentration.

Though with the present embodiment, activated carbon was used as anexample of an adsorbing material, a porous complex, zeolite, or otherporous material, which is suitably adjusted in pore diameter or whichhas suitable pore diameter, may be used instead. A three-dimensionalmetal complex of a dicarboxylic acid, etc., may be given as an exampleof a porous complex.

Second Embodiment

With this embodiment, an isotopic gas is separated and enriched bysealing a mixed gas once inside an adsorption chamber in which anadsorbing material is stored and thereafter taking out the mixed gasthat flows from the adsorption chamber after the elapse of apredetermined time from the start of outflow of the mixed gas. As withthe first embodiment, this embodiment also uses the phenomenon that incomparison to a first gas, a specific isotopic gas is less readilyadsorbed onto a specific adsorbing material and is less readily desorbedfrom that adsorbing material.

With this embodiment, ¹²CH₄ is used as the first gas, ¹³CH₄ is used asthe isotopic gas to be separated, and activated carbon is used as theadsorbing material.

This embodiment uses the system shown as an example in FIG. 1. Here, acase of separating ¹³CH₄, which is the isotopic gas of ¹²CH₄, fromhigh-purity methane gas shall be described. Also, with this embodiment,an example of use of the same activated carbon as that of the firstembodiment as the adsorbing material shall be described.

FIG. 3 is a flowchart showing an example of a treatment procedure of anembodiment to which this invention's isotopic gas separation method isapplied.

First, in a state where all valves are closed, valve 112 is opened,exhaust pump 113 is made to operate, and adsorption chamber 107 is putin a state of reduced pressure. Valve 112 is then closed and valve 102is then opened to fill the interior of adsorption chamber 107 withhelium gas. Valve 102 is then closed and valve 112 is opened withexhaust pump 113 being in operation to exhaust the helium gas insideadsorption chamber 107. This series of operations is repeated aplurality of times to remove impurities that exist in adsorption chamber107 as much as possible and regenerate activated carbon 105 at the sametime. Adsorption chamber 107 is then put in a high vacuum state of 13 Paor less. Recovery tank 114 is also put in a high vacuum state.

From this state, the separation of the isotopic gas, in this case, theseparation of ¹³CH₄, which is the isotopic gas of ¹²CH₄, is started(step 601). First, with valve 112 being closed, valve 102 and valve 104are opened to introduce helium gas from piping 101 and high-puritymethane gas from piping 103 into adsorption chamber 107. In thisprocess, flow regulating devices 400 and 401 are adjusted to make thehelium gas and high-purity methane gas flow into adsorption chamber 107until the interior of adsorption chamber 107 reaches a predeterminedpressure. The flow rates of the methane gas and helium gas are set forexample to a ratio of 1:9. When the interior of adsorption chamber 107is filled with the helium and high-purity methane gas and thepredetermined internal pressure is attained, valves 102 and 104 areclosed to obtain a state in which the helium and high-purity methane gasare sealed inside adsorption chamber 107 (step 602). In this process,temperature regulating device 108 is made to operate to maintain thetemperature inside adsorption chamber 107 at a fixed value (for example,278K).

The time for which the high-purity methane gas is kept sealed insideadsorption chamber 107 is not less than a time with which ¹³CH₄ willbecome adequately adsorbed onto activated carbon 105. The time for whichthe high-purity methane gas is kept sealed inside adsorption chamber 107is set for example to 500 seconds.

After sealing the high-purity methane gas inside adsorption chamber 107for the predetermined time, valve 112 is opened and the gas that wassealed inside adsorption chamber 107 is exhausted out of the system(step 603). Then after the elapse of a predetermined time from the startof outflow of gas, valve 112 is closed and valve 111 is opened. Thehigh-purity methane gas that was sealed inside adsorption chamber 107 isthus taken out at a certain point in time and recovered in recovery tank114 (step 604). Here, the predetermined time from the start of outflowis set, for example, to 50 seconds.

When the taking out of the high-purity methane gas from adsorptionchamber 107 into recovery tank 114 is ended, valve 111 is closed.

In the state in which the high-purity methane gas is sealed insideadsorption chamber 107, ¹³CH₄ and ¹²CH₄ become adsorbed onto activatedcarbon 105. Then when the high-purity methane gas that was sealed insideadsorption chamber 107 is taken out, first ¹²CH₄ begins to desorb fromactivated carbon 105 and ¹³CH₄ begins to desorb from activated carbon105 at a delayed timing. Thus if outflowing gas is recovered after theelapse of a predetermined time from the start of outflow of thehigh-purity methane gas from adsorption chamber 107, the abundance ratioof ¹³CH₄, which is delayed in desorption, in the outflowing gas will behigher than that of ¹²CH₄, which had become desorbed earlier.High-purity methane gas, which is enriched in ¹³CH₄ is thus collected inrecovery tank 114.

The high-purity methane gas collected in recovery tank 114 is sent assuited to distillation column 131 by the function of flow regulatingdevice 118 and the helium gas is separated there. The high-puritymethane gas that has been separated from the helium gas is guided viapiping 133 from distillation column 131 to distillation column 141 andis subject to further enrichment of ¹³CH₄. As with the first embodiment,131 may be a gas separation device, such as a PSA. Since the process atdistillation column 141 is the same as that of the first embodiment, adescription thereof shall be omitted.

Next, with exhaust pump 113 being in operation, valve 112 is opened andthe interior of adsorption chamber 107 is put in a high vacuum state.

In this process, temperature regulating device 108 may be controlled toheat the interior of adsorption chamber 107 to enhance the regenerationefficiency. The heating temperature is set, for example, to 373K. Valve102 is then opened to make helium gas flow and the ¹²CH₄ and ¹³CH₄ thathad become desorbed from activated carbon 105 are discharged fromadsorption chamber 107. The regeneration process is thus executed (step605).

Normally after the regeneration process, the judgment of repeating theprocess of separating ¹³CH₄ again is made (step 606), and the methanegas is introduced inside adsorption chamber 107 again and the ¹³CH₄separation process of the next cycle is performed. The ¹³CH₄ separationprocess using activated carbon 105 and the regeneration process ofactivated carbon 105 are thus performed repeatedly to send methane gas,which has been made high in ¹³CH₄ concentration, to distillation column131.

A process of not using recovery tank 114 may also be carried out in thepresent embodiment as well.

The concentration of ¹³CH₄ at the stage of introduction intodistillation column 141 is set to 10 volume % or more in the presentembodiment as well.

To end the ¹³CH₄ separation process, a “no” judgment is made in step 606and the separation of the isotopic gas using adsorption is ended (step607). Also, the respective steps described above may be executedautomatically in accordance with a priorly prepared program using anunillustrated computer control device, etc.

The same advantage of enabling separation of ¹³CH₄ at low cost, providedby the first embodiment, is also provided by the present embodiment.

With the present embodiment, the piping that is used to make methane gasflow into adsorption chamber 107, which is the adsorption chamber, andthe piping that is used to discharge methane gas from adsorption chamber107 may be the same piping.

Though with the above-described embodiment, activated carbon was used asan example of an adsorbing material, a porous complex, zeolite, or otherporous material, which is suitably adjusted in pore diameter or whichhas suitable pore diameter, may be used instead. A three-dimensionalmetal complex of a dicarboxylic acid, etc., may be given as an exampleof a porous complex.

Third Embodiment

This embodiment is an example of use of the phenomenon that, with aspecific adsorbing material and a specific mixed gas, the adsorptiononto the adsorbing material occurs relatively more readily with theisotopic gas than the first gas. That is, a mixed gas of the first gasand the isotopic gas is made to flow in and pass through an adsorptionchamber, the inflow and outflow of the mixed gas into and from theadsorption chamber is stopped at a stage at which a predetermined timehas elapsed, and thereafter, the isotopic gas that had becomeselectively adsorbed onto the adsorbing material in the adsorptionchamber is taken out from inside the adsorption chamber. Here, sincerelatively more of the isotopic gas is adsorbed onto the adsorbingmaterial than the first gas, the concentration of the isotopic gas inthe mixed gas that is taken out from the adsorption chamber will behigher than the concentration in the mixed gas prior to introductioninto the adsorption chamber. And at stage at which the concentration ofthe isotopic gas has been increased to some degree, the method isswitched to distillation to perform further separation and enrichment ofthe isotopic gas. The isotopic gas is thereby obtained at high purity.

With the present embodiment, ¹²CO is used as the first gas, ¹³CO is usedas the isotopic gas to be separated, and zeolite is used as theadsorbing material.

This embodiment shall now be described using the system illustrated inFIG. 1. Helium (He), to be used as carrier gas, is introduced from apiping 101. High-purity carbon monoxide gas (CO), which is the mixedgas, is introduced from a piping 103. Adsorbing material 105 is azeolite-based adsorbing material, and for example, faujasite zeolite isused. The adsorbing material is housed in an adsorption chamber 107. Theinterior of adsorption chamber 107 can be adjusted to an arbitrarytemperature by means of a temperature regulating device 108. 114 is arecovery tank, which temporarily stores the exhaust from insideadsorption chamber 107. 131 is a distillation column for separatinghelium, which is the carrier gas, from the carbon monoxide gas. As withthe first embodiment, 131 may be a gas separation device, such as a PSA.The function of distillation column 141 is the same as that describedwith the first embodiment.

Helium is used as the carrier gas for the process of making ¹³CO, whichis the isotopic gas, become adsorbed onto adsorbing material 105. Heliumis also used as the carrier gas for recovering the isotopic gas ¹³COthat had become adsorbed onto adsorbing material 15. By making helium,which is the carrier gas, flow, the ¹³CO that had become adsorbed ontoadsorbing material 105 is desorbed and the ¹³CO is recovered along withthe helium.

FIG. 5 is a flowchart showing an example of a treatment procedure of anembodiment to which this invention's isotopic gas separation method isapplied. First, prior to treatment, an exhaust pump 113 is made tooperate with all valves except for a valve 112 being closed to bring theinterior of adsorption chamber 107 to a high vacuum state. Valve 112 isthen closed and a valve 102 is opened to fill the interior of adsorptionchamber 107 with helium gas. This process is repeated several times toremove impurities from the interior of adsorption chamber 107 andheighten the adsorption capacity of adsorbing material 105. Recoverytank 114 is also put in a high vacuum state.

The separation of isotopic gas is started with adsorption chamber 107being in a high vacuum state (step 701). First, with adsorption chamber107 being in a high vacuum state, valve 101 and valve 104 are opened.Here, flow regulating devices 400 and 401 are made to operate so thathelium and carbon monoxide gas will flow into adsorption chamber 107 atproportions, for example, of 80 volume % and 20 volume %, respectively.The supply of mixed gas is thus started (step 702).

When the pressure inside adsorption chamber 107 reaches atmosphericpressure as a result of the abovementioned supply of helium gas andcarbon monoxide gas, valve 112 is opened and while adjusting flowregulating devices 400 and 401 to maintain the pressure insideadsorption chamber 107 at atmospheric pressure, the supply of helium gasand carbon monoxide gas is continued. A state in which helium gas andcarbon monoxide gas pass through the interior of adsorption chamber 107is thus created. The supply of helium gas and carbon monoxide gas iscontinued, for example, for 200 seconds. The pressure inside adsorptionchamber 107 may be maintained at a pressure other than atmosphericpressure.

When carbon monoxide gas is supplied, since ¹³CO is more readilyadsorbed by adsorbing material 105, comprising zeolite, than ¹²CO, moreof the ¹³CO will become adsorbed to adsorbing material 105. Thusinitially, carbon monoxide gas with which the concentration of ¹³CO islow will be exhausted from adsorption chamber 107. When a certain amountof flow has passed through adsorbing material 105, the adsorptionequilibrium state is reached and enrichment by selective adsorption of¹³CO will no longer occur. The ratio of ¹²CO to ¹³CO in the carbonmonoxide gas that is exhausted from adsorption chamber 107 will thenbecome the same as that of the stage prior to introduction of the carbonmonoxide gas into adsorption chamber 107.

After making helium gas and carbon monoxide gas flow for a predeterminedamount of time, valve 102, valve 104, and valve 112 are closed (step703). Valve 111 is then opened and the gas inside adsorption chamber 107is recovered by recovery pump 110 into recovery tank 114. In thisprocess, the temperature inside adsorption chamber 107 may be raised,for example, to 423K by means of temperature regulating device 108 toenhance the recovery efficiency. Since adsorption chamber 107 is put ina relatively depressurized state and is heated in this process, thecomponents that had become adsorbed onto adsorbing material 105 becomedesorbed and are recovered in recovery tank 114 (step 704). With thesedesorbed components, the value of ¹³CO/¹²CO is greater than that at thestate of introduction into adsorption chamber 107. Carrier gas may bemade to flow into adsorption chamber 107 in this process to increase theefficiency of recovery.

Next, valve 111 is closed and valve 102 and valve 112 are opened to makehelium gas flow into adsorption chamber 107. At this time, thetemperature inside adsorption chamber 107 may be raised, for example, at423K to enhance the regeneration efficiency. Further desorption of thecarbon monoxide gas that had become adsorbed onto adsorbing material 105is thereby carried out to regenerate adsorbing material 105 (step 704).By regeneration, the adsorption performance of adsorbing material 105 isrevived. Since the ¹³CO concentration of the exhaust gas from adsorptionchamber 105 in this process is higher than the natural concentration,this exhaust gas may also be recovered in recovery tank 114.

The ¹³CO concentration of the recovered carbon monoxide gas is higherthan the natural concentration. Carbon monoxide gas, which has thus beenenriched in the ¹³CO component, is thus collected in recovery tank 114.The helium gas, which is the carrier gas, is recovered along with thecarbon monoxide gas in the recovery tank. Also, without providingrecovery tank 114, flow regulating device 118 (or a suitable pump) maybe used to intermittently send the carbon monoxide gas, with which the¹³CO component has been enriched, to distillation column 131.

In the case where separation of the isotopic gas using adsorption is tobe continued, a “yes” judgment is made at step 705 and a return to step702 is performed. In this case, the interior of adsorption chamber 107is put in a reduced pressure state again and the procedures from step702 onwards are repeated.

The carbon monoxide gas and helium gas that are stored in recovery tank114 are sent to distillation column 131 by operation of flow regulatingdevice 118. Separation of helium gas and carbon monoxide gas is carriedout at distillation column 131. Since helium gas and carbon monoxide gasdiffer greatly in boiling point, practically complete separation ofhelium gas and carbon monoxide gas is carried out at distillation column131. Here, the carbon monoxide gas, which is the high-boiling-pointcomponent, collects at the lower part of distillation column 131 and issent to distillation column 141 via piping 133. The helium gas, which isthe low-boiling-point component, is collected at the upper part ofdistillation column 131 and is recovered from piping 132.

The carbon monoxide gas that is supplied to distillation column 141 isput in a state where the ¹³CO component has been enriched to at least 10volume %. In actuality, the above-described ¹³CO separation work usingadsorption is carried out in the necessary number of stages until the¹³CO component is enriched to at least 10 volume %.

The carbon monoxide gas, with which the ¹³CO component has been enrichedto at least 10 volume %, is supplied via piping 133 to distillationcolumn 141. Then at distillation column 141, the process of separating¹³CO, which is the high-boiling-point component, and ¹²CO, which is thelow-boiling-point component, is carried out. Since the process atdistillation column 141 is the same as that described with the firstembodiment, a description thereof shall be omitted.

With the method of the third embodiment, activated carbon, silica gel,or alumina may be used as the adsorbing material. As a zeolite-basedadsorbing material, faujasite, pentasil zeolite, mordenite, or A-typezeolite may be used.

Fourth Embodiment

With this embodiment, an isotopic gas enrichment process by distillationis compared with cases where an isotopic gas enrichment process bydistillation is combined with an isotopic gas enrichment process byadsorption in order to explain the advantages of the art by thisinvention.

FIG. 4 shows process diagrams for cases of using high-purity carbonmonoxide gas, containing ¹³CO at the natural abundance ratio, as the gasto be treated and attempting to obtain ¹³CO of a purity of 99 volume %by (a) distillation, (b) adsorption+distillation (1), and (c)adsorption+distillation (2).

With (a), a five-column arrangement was used in regard to distillationcolumns, with each distillation column being 0.15 m in inner diameterand having 750 stages. The heat quantity required for the reboilers ofthe distillation columns was 35 kW for the entirety in this case. Also,the holdup volume, which depends on the number of distillation columns,was 2 m³ in this case.

With (b), the first distillation column of (a) was replaced by anadsorption process, and the method of third embodiment described abovewas used for this adsorption process. Here, the ¹³CO concentration atthe exit of the adsorption process (entrance of the distillationprocess) was 10 volume %, and a two-column arrangement, withdistillation columns of the abovementioned inner diameter and number ofstages, was used for the distillation process. The heat quantityrequired for the reboilers is proportional to the number of distillationcolumns and was thus reduced to 14 kW and the holdup volume was reducedto 0.8 m³ in comparison to (a).

With (c), the first and second distillation columns of (a) were replacedby an adsorption process, and the method of third embodiment describedabove was used for this adsorption process. Here, the ¹³CO concentrationat the exit of the adsorption process (entrance of the distillationprocess) was 45 volume %, and a single-column arrangement, with adistillation column of the abovementioned inner diameter and number ofstages, was used for the distillation process. The heat quantityrequired for the reboiler is proportional to the number of distillationcolumns and was thus reduced to 7 kW and the holdup volume was reducedto 0.4 m³ in comparison to (a).

The above comparison shows that an isotopic gas of a predeterminedconcentration can be obtained while reducing the heat quantity requiredfor the reboilers and using less energy (heating quantity for heatingrequired at the reboilers and heating quantity for cooling required atthe condensers). That is, whereas in the case (a) of using justdistillation, three distillation columns were required for the initialstage (first stage) of enrichment and a large consumption energy (forexample, consumption power) is required there, by combining with anadsorption process, these three distillation columns can be eliminated.By thus combining an adsorption process at a stage prior to thedistillation process, the load on the enrichment process at a stage oflower concentration can be reduced and low consumption energy (forexample, power savings) can be achieved.

Also in regard to the period from the start of operation of theequipment to steady-state operation (startup period), since with anadsorption process, an enrichment stage is not required and the startupperiod can be ignored, the startup period of the entire process dependson the holdup volume of the distillation columns and if the startupperiod of (a) is set to 1, it is reduced to 0.9 in the case of (b) andto 0.3 in the case of (c). Steady-state operation refers to theoperation state at which ¹³CO of predetermined concentration is obtainedsteadily.

The results of the above described comparison clearly show that thestartup period can be reduced by combining an adsorption process at alow enrichment stage (initial stage of the enrichment process). Thismeans that the period from the start of operation of a plant to theshipment of a product can be shortened and gives rise to such economiceffects as early recovery of invested capital. The means is thusextremely effective for cases where, for example, an existing plant isto be reinforced to increase the shipment amount in a short period oftime.

Also as is clear from FIG. 4, by incorporating an adsorption process ata low enrichment stage (initial stage of the enrichment process in thiscase), the investment amount necessary for equipment can be reduced. Insuch a case where carbon monoxide gas is selected as the mixed gas, byperforming separation by adsorption until the concentration of ¹³CO gasin the mixed gas becomes 10 volume % and thereafter switching toseparation by distillation, the number of distillation columns can bereduced from five to three in comparison to a case where justdistillation is performed and the consumption energy can also be reducedgreatly accordingly. Furthermore, by performing separation by adsorptionuntil the concentration of ¹³CO gas in the mixed gas becomes 45 volume %and thereafter switching to separation by distillation, further loweringof the consumption energy can be achieved. It can thus be said that forseparation of ¹³CO gas using carbon monoxide gas, the use of anadsorption method until the concentration of ¹³CO gas in the mixed gasbecomes 10 volume % (or more) is preferable and the use of an adsorptionmethod until the concentration of ¹³CO gas in the mixed gas becomes 45volume % (or more) is even more preferable.

Though this invention has been described specifically based onembodiments above, this invention is not limited to the aboveembodiments and may be modified within a scope that does not deviatefrom the gist of the invention.

With the above-described embodiments of this invention, the separation(enrichment) of the isotopic gas by adsorption may be carried out inseveral stages or in plurality in parallel. Also, a plurality ofseparation devices may be prepared to enable the isotopic gas separationprocess using adsorption to be carried out in plurality in parallel andthe treatment timing of the respective separation devices may be shiftedsuitably so that mixed gas, with which the isotopic gas concentrationhas been increased, will be supplied continuously to the isotopicseparation process using distillation.

1. An isotopic gas separation method for separating, from a mixed gascontaining a molecular or atomic first gas, an isotopic gas of saidfirst gas, comprising: one of either a first treatment procedure, inturn comprising the steps of: supplying said mixed gas to a gas inlet ofan adsorption chamber; and taking out the isotopic gas of said first gasthat flows out from a gas outlet of said adsorption chamber from thestart of supplying of said mixed gas to the point of elapse of apredetermined time; or a second treatment procedure, in turn comprising:a first step of sealing said mixed gas in an adsorption chamber; asecond step of making said mixed gas flow out from said adsorptionchamber after said first step; and a third step of taking out theisotopic gas of said first gas after the elapse of a predetermined timefrom the start of said second step; and further comprising: a treatmentprocedure of enriching, by distillation, the isotopic gas of said firstgas that has been taken out.
 2. The isotopic gas separation method asset forth in claim 1, wherein a porous material is set as a material foradsorbing said isotopic gas in said adsorption chamber.
 3. The isotopicgas separation method as set forth in claim 2, wherein said porousmaterial is activated carbon, zeolite, or a complex.
 4. The isotopic gasseparation method as set forth in claim 1, wherein said mixed gas ismethane gas or ammonia gas.
 5. The isotopic gas separation method as setforth in claim 1, wherein the concentration of said isotopic gas isincreased at least to a concentration exceeding the natural abundanceratio through said first treatment procedure or second treatmentprocedure.
 6. An isotopic gas separation method for separating, from amixed gas containing a molecular or atomic first gas, an isotopic gas ofsaid first gas, comprising the steps of: supplying said mixed gas to agas inlet of an adsorption chamber; stopping said supply after theelapse of a predetermined time from the start of supply of said mixedgas; relatively depressurizing the interior of said adsorption chamberand taking out said mixed gas that had become adsorbed inside saidadsorption chamber; and enriching, by distillation, the isotopic gas ofsaid first gas contained in said mixed gas that has been taken out. 7.The isotopic gas separation method as set forth in claim 6, wherein aporous material is set as a material for adsorbing said isotopic gas insaid adsorption chamber.
 8. The isotopic gas separation method as setforth in claim 7, wherein said porous material is zeolite, activatedcarbon, silica gel, or alumina.
 9. The isotopic gas separation method asset forth in claim 6, wherein said mixed gas is carbon monoxide gas. 10.The isotopic gas separation method as set forth in claim 6, wherein, ina stage prior to said step of enriching by distillation, theconcentration of said isotopic gas is at least a concentration exceedingthe natural abundance ratio.
 11. An isotopic gas separation method forseparating, from a mixed gas containing a molecular or atomic first gas,an isotopic gas of said first gas, comprising the steps of: supplyingsaid mixed gas to a gas inlet of an adsorption chamber; stopping saidsupply after the elapse of a predetermined time from the start of supplyof said mixed gas; making a carrier gas flow inside said adsorptionchamber and taking out, along with said carrier gas, said mixed gas thathad become adsorbed inside said adsorption chamber; and enriching, bydistillation, the isotopic gas of said first gas contained in said mixedgas that has been taken out.
 12. The isotopic gas separation method asset forth in claim 11, wherein a porous material is set as a materialfor adsorbing said isotopic gas in said adsorption chamber.
 13. Theisotopic gas separation method as set forth in claim 12, wherein saidporous material is zeolite, activated carbon, silica gel, or alumina.14. The isotopic gas separation method as set forth in claim 11, whereinsaid mixed gas is carbon monoxide gas.
 15. The isotopic gas separationmethod as set forth in claim 11, wherein, in a stage prior to said stepof enriching by distillation, the concentration of said isotopic gas isat least a concentration exceeding the natural abundance ratio.
 16. Anisotopic gas separation device for separating, from a mixed gascontaining a molecular or atomic first gas, an isotopic gas of saidfirst gas, comprising: an adsorption chamber, the interior of which canbe set to a depressurized state of a pressure lower than atmosphericpressure; a gas exhausting means, exhausting gas from said adsorptionchamber; a gas supply port, supplying gas into said adsorption chamber,and a gas discharge port, discharging gas out from said adsorptionchamber, or a gas supply/discharge port, supplying and discharging gasto and from said adsorption chamber; a single or a plurality of valvesor a gas flow controlling means, controlling the supply, discharge,sealing, or supply flow rate of gas with respect to said adsorptionchamber or controlling the gas pressure inside said adsorption chamber;a porous material, set inside said adsorption chamber, as a material foradsorbing said isotopic gas; and a distillation device, disposed at astage subsequent said gas discharge port or gas supply/discharge portand distilling the discharged gas taken out from said gas discharge portor gas supply/discharge port.
 17. The isotopic gas separation device asset forth in claim 16, wherein said mixed gas is methane gas, ammoniagas, or carbon monoxide gas.
 18. The isotopic gas separation device asset forth in claim 16, wherein said porous material is activated carbon,zeolite, a complex, silica gel, or alumina.
 19. The isotopic gasseparation device as set forth in claim 16, further comprising a meansfor heating said adsorbing material inside said adsorption chamber. 20.An isotopic gas separation method for separating, from a mixed gascontaining a molecular or atomic first gas, an isotopic gas of saidfirst gas, comprising the steps of: making said mixed gas contact anadsorbing material with selectivity in the adsorption of said first gasand said isotopic gas and obtaining a mixed gas with which said isotopicgas is enriched; and enriching the concentration of said isotopic gasfurther by distillation.
 21. An isotopic gas separation device forseparating, from a mixed gas containing a molecular or atomic first gas,an isotopic gas of said first gas, comprising: an adsorption chamber, inwhich said mixed gas is made to flow or is sealed and having installedtherein an adsorbing material with selectivity in the adsorption of saidfirst gas and said isotopic gas; and a distillation device, distillingthe discharged gas taken out from said adsorption chamber.